When the 27-year-old Spaniard won the 2010 U.S. Open title he uncorked a 135-mph bomb — perhaps the fastest of his career — and averaged 10-12 mph more on his first and second serves than the year before. (The reason, he said, was a subtle grip change he made a few days before the tournament)

This year, his fastest serve is 124 mph. His average speeds compared with 2010 are down too — by a lot.

Through four rounds, he has dipped from 119 mph to 108 mph on his first serve and 107 mph to 87 mph on his second. Instead, Nadal — the best problem solver in the sport — has improved the accuracy and variety, mixing it up and and finding new areas of the service box to exploit.

"He's not as predictable as he used to be," says 1987 Wimbledon champion Pat Cash.

According to Leo Levin, who oversees U.S. Open statistics for IBM, left-hander Nadal's wide serve to the ad court has become more effective. He's also crowding opponents with body serves and threading it up the middle from either side.

"Nadal is now doing it with placement as opposed to raw speed," Levin wrote in an email. "He's also figured out that his high-kicking second serve creates more issues for his opponents than if he goes for more speed and less spin."

If he is dialing back his power on serve, Nadal has become more aggressive in the rest of his game. He's coming forward more willingly to finish points when he gets a short ball or at the net. He's won 71 of 89 net points this year.

Why sacrifice power for accuracy? Because Nadal has one of the best one-two punches in tennis. More first serves means fewer strong returns, which allow Nadal to use his forehand to gain control of points.

"He's going for precision," No. 1 Novak Djokovic said Tuesday after manhandling Marcel Granollers of Spain 6-3, 6-0, 6-0. "He knows that he can back it up. He has one of the best second shots in the game."

Statistics back Djokovic up: Nadal is the only player on the ATP Tour averaging more than 70% both in first serve percentage and points won on first serve.

Nadal said Monday after beating No. 22 seed Philipp Kohlschreiber of Germany in four sets that "my serve is working just amazing."

But the eight-time French Open winner said it was more his play from the backcourt that helped him remain unbroken in the tournament.

Still, he has gone 56 for 56 on serve and is the only quarterfinalist not to drop serve. (In fact, he is on a 71-game holding streak going back to his match against Tomas Berdych in the Cincinnati Masters last month).

"I think I didn't lose the serve because I played good points from the baseline," said Nadal, who faces fellow Spaniard and No. 19 seed Tommy Robredo of Spain in tonight's quarterfinals.

As usual, Nadal has found a way — despite returning from a seven-month layoff for his ailing left knee — to elevate his game on what has been his most troublesome surface.

"It's been an incredible comeback," said Cash, who also does TV and newspaper commentary. "We all thought the hardcourts were going to be worst on his body. He keeps improving his game."

Nadal, no stranger to quirks and superstitions, should add this one: The last time he went four rounds in New York without losing serve? His championship run in 2010.

GALLERY: U.S. OPEN DAY 9

Stanislas Wawrinka celebrates after winning a point during his match against Tomas Berdych. (Photo: Jerry Lai, USA TODAY Sports)

Peng Shuai, of China, right, serves as Hsieh Su-Wei, of Taiwan, looks on during a third round doubles match against Jelena Jankovic, of Serbia, and Mirjana Lucic-Baroni, of Croatia. (Photo: Kathy Willens AP)